Add options for controlling the number of connections

The options 'connections_per_host' (default 100) and 'threads_allowed_to_block_for_connection_multiplier' (default 5)
can be used to control how many connections are used at most for a single river.

The defaults are based on the values in MongoClientOptions.Builder for the 2.12.4 mongo-java-driver.
Merge pull request #371 from Collaborne/conn-options

Add options for controlling the number of connections